1. Field of the Invention
The present invention relates to a paste for forming an internal electrode in a ceramic element such as a laminated ceramic capacitor.
2. Description of the Related Art
Among ceramic elements, laminated ceramic capacitors are generally produced by the following steps: application, through screen printing, of a paste containing Pd or Ag/Pd as a primary component for forming an internal electrode onto a ceramic green sheet formed of a ceramic dielectric material comprising barium titanate, etc. as a primary component and having a thickness of 10-30 .mu.m; laminating a predetermined number of the printed sheets; pressing; cutting; and firing at about 900-1400.degree. C. in air.
Ceramic green sheets that are used in the above process are produced by mixing ceramic powder and an organic binder to provide a powder-dispersed slurry, molding the slurry into a sheet by the doctor blade method, and drying to an appropriate level of dryness.
Pastes for forming an internal electrode that are used in the above process are produced by incorporating Pd powder, Ag-Pd alloy powder, or a powder mixture of Ag and Pd into an organic vehicle by way of mixing to thereby achieve dispersal.
In general, firing for the production of the above-described laminated ceramic capacitors comprises two steps, i.e., a degreasing step wherein the binder contained in the ceramic green sheets and a binder component contained in the paste for forming an internal electrode are combusted for decomposition, and a firing step wherein the ceramic powder and the metal powders are sintered.
In the degreasing step, the binder component in the paste suddenly generates heat at relatively low temperature (about 200.degree. C.) due to a catalytic effect of the metal powder serving as a material for an internal electrode. Therefore, the step involves a problem that structural defects such as cracks and delamination occur in a dielectric layer of the laminated ceramic capacitors.
In the firing step, the Pd or an Ag/Pd component in the internal electrode is oxidized during firing in air, particularly at about 300.degree. C.-800.degree. C., to thereby expand. This step involves a problem that the oxidation-induced expansion generates internal stress in the ceramics to easily cause structural defects such as delamination.